Extracellular matrix (ECM)-degrading proteinases of the matrix metalloproteinase (MMP) gene family have been implicated in the pathogenesis of breast neoplastic progression, yet also play a role in normal tissue remodeling. This project tests the hypothesis that MMPs are essential contributors to the interaction between mammary epithelial cells, the adipogenic stroma and the extracellular matrix. In the previous grant period, we used genetic analysis in transgenic mice and mammary cultures, to show that MMPs expressed by stromal cells and mammary epithelial cells alter the cellular microenvironment in the mammary gland during development and lead to neoplastic progression. The present proposal addresses the molecular mechanisms by which these MMPs orchestrate the cross-talk between the mesenchymal and epithelial compartments. In parallel, the genetic approach uses in vivo models and cell, organoid and organotypic cultures to analyze mammary gland development and dysfunction. The effects of altering MMP levels and activity on the phenotype and behavior of mammary epithelial progenitor and ductal cells and stromal cells will be studied in transgenic mice and in culture models. Morphogenesis, differentiation, and migratory and invasive behavior will be studied in separated and interacting populations of cells in various microenvironmental configurations. In comparison, MMP expression in animals with altered branching morphogenesis induced by oncogenes. The MMPs that mediate these processes and their molecular targets in the extracellular microenvironment or on the cell surface will be elucidated. The mechanisms by which MMPs stimulate ductal proliferation, branching and lobuloalveolar development will be compared and contrasted with the mechanisms by which they regulate involution in the post-lactational mammary gland. In both cases the contribution of both mammary epithelial and stromal cells will be evaluated. These experiments will elucidate how MMPs alter the cellular microenvironment, and what the critical responses are in normal and abnormal mammary gland epithelium and stroma. These studies address an important aspect of women's health, that of how MMPs regulate normal and neoplastic mammary gland function. Understanding the normal biology of the MMP genes should give insights into how cancer usurps these mechanisms. Therefore, our studies should provide basic insights that will, in the long term, allow for development of better diagnosis and treatment of breast cancer.